The present invention relates to a method of manufacturing a polycrystalline semiconductor thin film for a thin film transistor used for a liquid crystal display panel etc., and an apparatus for manufacturing such a thin film.
Liquid crystal display devices are made thin in dimensions and light in weight, and have characteristics such as that they can be driven at a low voltage and a color display can be easily achieved. With such characteristics, they are recently used as display devices of, for example, personal computers and word processors. Of all the types of the liquid crystal display apparatus, a so-called active matrix type liquid crystal display device in which a thin film transistor (TFT) is provided for each and every pixel as a switching element, is presently the optimal display mode as a display apparatus for a full color television or an OA device. This is because even if the number of pixels is increased in the active matrix type apparatus, deterioration of the contrast, response and the like does not significantly occur, and further this type of apparatus is able to display a half tone (gray scale).
The active matrix type liquid crystal display apparatus has a structure in which two glass substrates (more specifically, an array substrate on which a plurality of pixel electrodes and transistors are formed in matrix, and a common electrode substrate opposing to the array substrate) and a liquid crystal layer interposed between these substrates.
More specifically, on one of the glass substrates, that is, on the common electrode substrate, color filters arranged to correspond to pixels, and transparent electrodes (common electrodes) are formed. On the other array substrate, pixel electrodes made of transparent electrodes arranged in matrix, and TFTs whose source electrodes are connected respectively to the pixel electrodes are provided. The gate electrodes of the TFTs are connected to address lines arranged in the X direction, and the drain electrodes thereof are connected to data lines arranged in a direction (Y direction) intersecting with the address lines at right angles.
In the liquid crystal display apparatus having the above-described structure, as an address signal and a data signal are applied respectively to address lines and data lines each at a predetermined timing, a voltage which corresponds to an image displayed, is applied to a respective pixel electrode. Thus, the alignment of liquid crystal, that is, the light transmittance, can be controlled on the basis of a potential difference between a common electrode and a pixel electrode, and thus a desired image can be displayed. For details, an article written by T. P. Brody et al., (IEEE Trans. On Electron Devices. Vol.ED-20. Nov. 1973. pp.995-1001) should be referred to.
As the semiconductor material for the conventional TFT, amorphous silicon or polycrystalline silicon is presently used. In particular, the active matrix-type liquid crystal display device, which employs polycrystalline silicon has a structure in which the drive circuit for applying a drive signal to gate lines and data lines, can be formed within the same substrate as the liquid crystal element. Therefore, the size of the display panel can be reduced, and the reliability of the connection between wiring lines can be made high.
FIG. 1 illustrates a method of forming a conventional polycrystalline silicon thin film. A laser beam output from an excimer laser device 209 is irradiated via a beam homogenizer 208 and an optical system 207, on an amorphous silicon layer 203 formed on a glass substrate 201, and thus the amorphous silicon thin film is annealed and transformed into a polycrystalline silicon film 202.
In order to improve the uniformity, a laser beam 205 is converted into a beam 204 having a slender and lengthy shape by the optical system 207 via the beam homogenizer 208, and then irradiated on the surface of the silicon thin film 203. The amorphous silicon layer 203 is melted by the laser energy, and then crystallized in the process of solidification. The laser pulse width is as short as 20 to 30 nanoseconds, and therefore amorphous silicon can be crystallized without increasing the temperature of the substrate much. For this reason, a glass substrate can be used as the substrate 201. As laser annealing is carried out while sending a stage 206 in the X direction at a sending pitch Sp and scanning it in the Y direction, a polycrystalline silicon thin film 202 can be formed on the entire surface of the substrate.
FIG. 2A is a plan view of the substrate 201. FIG. 2B shows an enlarged view of the section 2B in FIG. 2A. In the region irradiated with the lengthy beam 204, a polycrystalline silicon thin film 202 can be obtained. However, the melting/solidification phenomenon of the silicon thin film occurs extremely quickly, and also an enormous number of initial nuclei to grow to be polycrystalline silicon are present in the substrate. Consequently, the size of crystal grains thus obtained is as small as about 0.2 to 0.3 .mu.m. As a result, a great number of grain boundaries are created as can be seen in FIG. 2B, which give rise to boundaries for crystal grains 211. Therefore, it becomes difficult to obtain a TFT of a high mobility.
Therefore, it is inevitable that the drive circuit formed integrally with the periphery of the substrate of the liquid crystal display device should be designed with TFTs having a low mobility. Further, in order to process a high-speed display signal, it is necessary to use a plurality of parallel circuits and therefore the area of the drive circuit region increases.
In the case where a high speed operation is required, a special crystal silicon IC is provided on an outer side of the polycrystalline silicon TFT drive circuit. Further, the grain boundaries 12 contains a great number of defects, which causes dispersion of threshold voltage Vth of TFT, and therefore it is difficult to realize a high-efficiency analog circuit. Consequently, it is conventionally not possible to form a circuit necessary for digitally driving a liquid crystal display device, such as a digital-analog converter.
In order to improve the TFT characteristics, it is important to realize a method of preparing polycrystalline silicon having great crystal grains. FIG. 3A shows an example of such a method, in which a blank mask 213 is provided in an optical path of an irradiation beam 205 in order to create a temperature gradient within the surface of the polycrystalline silicon layer 202, thus making it possible to increase the size of the grains. It should be noted that in FIG. 3A, W.sub.ELA is a beam width, which is a shorter side of the lengthy beam.
In more detail, in the surface portion of the. polycrystalline silicon layer 202, located directly underneath a blank mask 213, a polycrystalline silicon region 242 located at about 1 .mu.m or less from the mask edge is melted due to the light beam coming around there. However, since the melting energy in the region 242 is low, the temperature of the region 242 becomes lower than that of the region 244, thus creating a large temperature gradient regionally in the surface of the silicon layer 203. As such a temperature gradient is created, the solidification starts from an area 243 having a lower temperature, and the crystal growth occurs from the area 243 as the starting point. Therefore, a larger grain diameter as compared to the case shown in FIG. 1 can be obtained; however this conventional technique was not very much practical due to the below-described drawback.
That is, since the temperature gradient is created regionally, the crystal grains grow only to have sizes of about 1 to 3 .mu.m. Further, the grain diameter increases only in the direction vertical to the mask edge, and in terms of the direction parallel thereto, the grain diameters are still as small as 0.2 to 0.3 .mu.m. In the case where the grain diameter is increased by scanning the substrate, the substrate sending pitch Sp should be set such that the mask edge (end) does not go beyond the closest grain boundary as shown in FIG. 3B, and the grain diameter must be made as small as about 0.5 .mu.m. When the entire surface of a substrate having a size of 300.times.400 mm is processed at such a pitch, it requires about 90 minutes under a laser oscillation of 300 Hz even if a beam having a length of 150 mm is used. Thus, the above-described technique requires a great amount of time for processing the entire surface of a large area substrate, and therefore the technique is not suitable for the manufacture of polycrystalline silicon.
Therefore, the application of a liquid crystal panel of the type in which a drive circuit made of polycrystalline silicon TFTs is built in, is limited to a small size liquid crystal such as of about 1 to 3 inches diagonally across, which does not require a high speed operation, that is, for example, a projection type liquid crystal display device. Thus, it is difficult to apply such a panel to a panel larger in size than that mentioned above.